Mold for Casting Polycrystalline Silicon Ingot, and Silicon Nitride Powder for Mold Release Material Thereof, Slurry Containing Silicon Nitride Powder for Mold Release Layer Thereof and Mold Release Material for Casting Thereof

ABSTRACT

Mold for casting a polycrystalline silicon ingot, and a silicon nitride powder for a mold release material thereof, a slurry containing a silicon nitride power for a mold release layer thereof, and a mold release material for casting thereof. The present invention relates to a silicon nitride powder for a mold release material of a mold for casting a polycrystalline silicon ingot characterized in that the percentage of primary particles of granular crystals monodispersed in powders is not less than 95% in terms of the area ratio calculated by analysis of an SEM image.

TECHNICAL FIELD

The present invention relates to a mold for casting a polycrystallinesilicon ingot, and a silicon nitride powder for a mold release materialthereof, a slurry containing a silicon nitride power for a mold releaselayer thereof and a mold release material for casting thereof.

BACKGROUND ART

A polycrystalline silicon has been widely used for one type ofsemiconductor substrates for forming a solar battery and the productionvolume thereof has been rapidly increased every year. Such apolycrystalline silicon is generally formed by pouring and solidifying asilicon melt heated to melt at high temperature into a mold of a quartzcrucible, a divisible graphite crucible or a quartz crucibleaccommodated into a graphite crucible, of which the inner surface ofwhich is applied with a mold release material using a spray, a brush ora spatula or by adding a silicon raw material to a mold followed bymelting once and then solidifying again.

The mold release material prevents a silicon ingot from contaminatingwith impurities or prevents a silicon melt from adhering to the innerwall surface of a crucible for casting, and plays an important role torelease a solidified silicon ingot from a mold. Such a mold releasematerial used includes a high-purity powder of silicon nitride, siliconcarbide, and silicon oxide or a mixed power thereof in terms ofgenerally a high melting point of the powder thereof and lowcontamination of a silicon ingot. In the past, many researches have beencarried out to develop a method of forming a mold release material layeron the inner surface of a mold, a method of producing a mold with suchtreatment, and a method of producing a silicon ingot using the moldthereof in order to increase the productivity of a silicon ingot.

For example, Patent Literature 1 describes that in production of asilicon ingot, peeling of parts of a mold release layer making a siliconmelt contact with a mold and contamination of a silicon melt with apeeled product are effectively prevented by eliminating in advanceaggregation of silicon nitride particles in a slurry using a siliconnitride powder SN-E10 made of Ube Industries, Ltd. produced by an imidethermal decomposition method, thereby uniformly adhering a slurryincluding silicon nitride particles coated with the oxide film to theinner surface of a mold base body so as to make the surface of the moldrelease layer flat, and using the mold with the mold release layerformed herewith for production of a silicon ingot.

Patent Literature 2 also describes, for example, a method of forming amold release layer having a two-layer structure in which the density isadjusted in order to provide both good adhesion to a mold and goodrelease properties of a silicon ingot, and a method of fusing the moldrelease material containing silicon oxide added thereto.

CITATION LIST Patent Literatures

-   Patent Literature 1: Japanese Patent Application 2008-532114 (WO    2008/026688 A1)-   Patent Literature 2: JP 2005-95924 A

SUMMARY OF INVENTION Technical Problem

However, in Patent Literature 1, though a silicon nitride powder SN-E10used has the specific surface area in a range of from 9.5 to 12.5 m²/gand the median particle diameter of the short axis of approximately 0.2μm determined by a scanning electron microscope (SEM) image, oxidationtreatment of the surface of a silicon nitride powder causes aggregationthereof to form an aggregated body with the median particle diameter ofapproximately 0.7 μm in a particle size distribution. To eliminate suchaggregation, a process of grinding a slurry after preparation isrequired, and further even if a slurry is ground for long hours in a wetball mill, the median particle diameter of particles in the slurrycannot be reduced to the original value of 0.2 μm in a particle sizedistribution and to the contrary, reaggregation of particles proceeds.

When a slurry is dispersed in water only by agitation without a grindingprocess and then applied by spraying, aggregated particles remain in amold release layer as they are, resulting in uneven distribution of thedense site and low density site in the mold release layer, and in thelow density site in the mold release layer, the bonding force amongpowders constituting the mold release material is small, therebyreducing the strength of a mold release material layer as well aslowering the adhesion to a mold to peel easily. There are such problemsas a mold release layer prepared by such a method is brittle and easilypeeled and broken, a silicon melt is penetrated into a low density sitein the mold release layer to adhere to the inner wall of a castingcrucible, thereby generating fragments when releasing a solidifiedsilicon ingot and reducing a yield.

In the method according to Patent Literature 2, a method of forming themold release layer having a two-layer structure with different densitiesand a method of adjusting the mold release layer by adding silicon oxideare complicated, and there are such problems as an increase in theprocess variables in production leads to higher costs and consistency inthe quality of a mold release layer is not high resulting in needs tosimplify the process, reduce costs, and improve the reliability ofquality of the mold release layer.

The present invention has been made in view of the foregoing problemsand it is an object of the present invention to provide a mold forcasting a polycrystalline silicon ingot, and a silicon nitride powderfor a mold release material thereof, a slurry containing a siliconnitride power for a mold release layer thereof, and a mold releasematerial for casting thereof, which can be obtained in low cost and haveexcellent adhesion to a mold, and prevents the formation of fragmentsand damage when releasing a solidified silicon ingot, forming a siliconingot with high quality and in high yield.

Solution to Problem

The present inventors conducted extensive studies to solve the foregoingproblems and as a result, have found that a specific silicon nitridepower is excellent as a mold release material for production of asilicon ingot, thereby leading to the present invention. That is, thepresent invention provides a silicon nitride powder for a mold releasematerial of a mold for casting a polycrystalline silicon ingotcharacterized in that the percentage of primary particles of granularcrystals monodispersed in powders is not less than 95% in terms of thearea ratio calculated by analysis of an SEM image.

The present invention also provides a slurry containing a siliconnitride powder for a mold release layer of a mold for casting apolycrystalline silicon ingot, in which the silicon nitride powder for amold release material of the mold for casting the polycrystallinesilicon ingot is dispersed in water, a mold release material forcasting, which contains a silicon nitride powder for a mold releasematerial, and a mold for casting a polycrystalline silicon ingot inwhich a mold release layer including the mold release material is formedon the inner surface thereof.

Advantageous Effects of Invention

As described above, the present invention can provide a mold for castinga polycrystalline silicon ingot, and a silicon nitride powder for a moldrelease material thereof, a slurry containing a silicon nitride powderfor a mold release layer thereof, and a mold release material for casingthereof, which can be obtained in low cost and have excellent adhesionto a mold, and prevents the formation of fragments and damage whenreleasing a solidified silicon ingot, forming a silicon ingot with highquality and in high yield.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a jig used in measuring the densityof a green compact of a crystalline silicon nitride powder pressed atuniaxial pressure.

FIG. 2( a) is a field emission scanning electron microscope (FE-SEM)photograph showing one example of a cross-section of a mold releaselayer formed by applying a silicon nitride powder.

FIG. 2( b) is a drawing obtained by tracing a contour of crystallinesilicon nitride particles in order to calculate the area ratio occupiedby monodispersed primary particles of granular crystals.

FIG. 3 is a photograph showing the peeling status of a mold releaselayer to study the peel strength of the mold release layer.

FIG. 4 is a cross-sectional photograph showing an example of the statusof a silicon melt penetrating into a mold in Example 3.

FIG. 5 is a cross-sectional photograph showing other example of thestatus of a silicon melt penetrating into a mold in Comparative Example4.

FIG. 6 is a photograph showing an example of the status of a mold and asilicon ingot when releasing the silicon ingot from a mold in Example 3.

FIG. 7 is a photograph showing other example of the status of a mold anda silicon ingot when releasing the silicon ingot from a mold inComparative Example 3.

FIG. 8 is a graph showing a volume-based particle size distribution,determined by the laser diffraction/scattering method, of raw materialpowders of amorphous silicon nitride before grinding obtained in aprocess of Example 1 and amorphous silicon nitride powders aftergrinding under the condition of Example 3 and Comparative Example 4.

DESCRIPTION OF EMBODIMENTS

A silicon nitride powder for a mold release material according to thepresent invention is a silicon nitride powder used as a mold releasematerial for a mold for casting a polycrystalline silicon ingot,characterized in that the percentage of primary particles of granularcrystals monodispersed in the silicon nitride powder is not less than95% in terms of the area ratio calculated by analysis of an SEM image.Granular crystals of a silicon nitride powder denote the particles whichare not needle-like crystals or columnar crystals but hexagonal crystalsand have the aspect ratio of no more than 1.5. Since a silicon nitridepowder for a mold release material according to the present inventionhas a higher ratio of a silicon nitride powder occupied in the moldrelease layer when the mold release layer is formed, a mold releaselayer densely packed with a high adhesive strength can be formed at lowcost on the inner wall of a mold for casting a silicon ingot, preventingthe formation of fragments and damage when releasing a solidifiedsilicon ingot to form a silicon ingot with high quality and in highyield.

Since a mold for casting a polycrystalline silicon ingot which uses aslurry containing a silicon nitride powder for a mold release layeraccording to the present invention to form a mold release layer andwhich contains a mold release material for casting uses a siliconnitride powder including a higher percentage of primary particles ofgranular crystals monodispersed in a mold release layer, the mold ischaracterized in that there are few particles such as needle-likecrystals or aggregated fine particles preventing densification of themold release layer in the case in which a conventional silicon nitridepowder is used, but the density of silicon nitride particles in the moldrelease layer is uniform and dense and the adhesive strength between themold release layer and a mold is high when the mold release layer isformed. Therefore, penetration of a silicon melt into a mold can beprevented to improve the mold releasing properties of a solidifiedsilicon ingot which can be obtained in high yield.

A silicon nitride powder for a mold release material according to thepresent invention is characterized by having a high density in a greencompact, and for example, the density of a green compact pressed at auniaxial pressure of 2 tons/cm² is from 51 to 57% of a theoreticalvalue. The density in a green compact pressed at a uniaxial pressure of2 tons/cm² is obtained by filling 0.65 g of the sample in a mold with aninner diameter of 13 φmm in the jig used for measuring the density of agreen compact prepared by a uniaxial pressure process according to FIG.1, pressing to a predetermined pressure in 30 seconds, and maintainingthe pressure for 10 seconds, followed by releasing the pressure anddetermining the volume thereof, and then calculating from the measuredvalue and a theoretical density of 3.186 g/cm³ for a silicon nitridepowder sample. When the percentage of monodispersed primary particles ofgranular crystals is below 95% in terms of the area ratio calculatedfrom analysis of an SEM image, the density of a green compact preparedby a uniaxial pressure process becomes below 51% if the primary particlediameter of the powder is fine. Therefore, it is undesirable since thedensity of the mold release layer is reduced, a bonding force amongpowders constituting the mold release layer is low, and the adhesivestrength of the mold release layer is decreased as well as adhesion to amold is reduced and the mold release layer is brittle and easily peeledand broken, and a silicon melt penetrates into the mold release layer toadhere to the inner wall of a casting crucible generating fragments whenreleasing a solidified silicon ingot, thereby lowering a yield. Thedensity of a green compact prepared by a uniaxial pressure process usinga silicon nitride powder of which the area ratio occupied bymonodispersed primary particles of granular crystals is not less than95% is 57% at maximum.

Generally, a silicon nitride powder which can form a green compact withthe high density of from 51 to 57% prepared by a uniaxial pressureprocess can be obtained by grinding a silicon nitride powder with agrinding machine such as an attritor followed by the refining process.However, most of the silicon nitride powder, which was subjected to therefining treatment, become aggregated particles and furthermore a largeamount of metallic impurities remain so that the silicon nitride powderis not preferred as a mold release material constituting a mold releaselayer.

A silicon nitride powder for a mold release material according to thepresent invention can be obtained by thermally decomposing anitrogen-containing silane compound obtained by reaction of a siliconhalide with ammonia to yield an amorphous silicon nitride powderfollowed by calcination for crystallization (imide thermal decompositionmethod) or by grinding and classifying a bulk of silicon nitride powdersobtained by direct nitridation of metallic silicon powders (directnitridation method).

As the imide thermal decomposition method, there are, for example,methods described in JP 2907366 A, JP 2907367 A, and JP 3282456 A, inwhich a nitrogen-containing silane compound described in JP 3077870 Acan be used as a raw material for an amorphous silicon nitride powder.

In production of a silicon nitride powder by the direct nitridationmethod, a bulk of a nitridated product has to be finely ground and longhours are required for grinding and the powder is contaminated with alarge amount of metallic impurities of a grinding media, and not lessthan several hundreds ppm of metallic impurities still remain even ifthe powder is treated with a mineral acid for refinement after grinding.In particular, the material with high hardness used in a grinding mediacontains a large amount of multivalent metals such as Cr and W nearlyinsoluble in a mineral acid used in refinement, thereby leaving a largeamount of them after refinement. Therefore, the imide thermaldecomposition method is superior as a method for preparing a siliconnitride powder for a mold release material used in the presentinvention.

A nitrogen-containing silane compound as a raw material for the imidethermal decomposition method includes silicon diimide (Si(NH)₂), silicontetramide, silicon nitride imide, and silicon chloro imide. Thesecompounds can be produced by a prior art method, for example, the methodof reacting in gas phase a silicon halide such as silicon tetrachloride,silicon tetrabromide, silicon tetraiodide with ammonia and a method ofreacting the liquid silicon halide with liquid ammonia. An amorphoussilicon nitride powder is also produced by the prior art method, forexample, a method of thermally decomposing the nitrogen-containingsilane compound under an atmosphere of the nitrogen or ammonia gas at atemperature range of from 1,200 to 1,460° C. and a method of reacting asilicon halide such as silicon tetrachloride, silicon tetrabromide, andsilicon tetraiodide with ammonia at high temperature.

The particle size and particle shape of a silicon nitride powder for amold release material according to the present invention can becontrolled by, for example, adjusting in the imide thermal decompositionmethod the heating temperature of thermal decomposition in preparationof an amorphous silicon nitride powder and the crystallization rate withan oxygen content and adjusting the particle growth rate with thetemperature elevation rate in calcination for crystallization. It ispreferred that the specific surface area value of monodispersed primaryparticles of granular crystals is adjusted to be from 0.5 to 13 m²/g. Ina silicon nitride powder with the specific surface area value below 0.5m²/g, the percentage of aggregated particles which are fused together isincreased. To prepare a silicon nitride powder with the specific surfacearea exceeding 13 m²/g, the oxygen content in an amorphous siliconnitride powder has to be increased. Therefore, particulates of a siliconnitride powder with the specific surface area exceeding 13 m²/g is notpreferred, since an increase of the aggregation force decreases thepercentage of monodispersed primary particles of granular crystals aswell as an increase of the oxygen content increases the reactivity witha silicon melt.

Further, a silicon nitride powder for a mold release material accordingto the present invention can be obtained by reducing the amount of thecoarse aggregated particles in a nitrogen-containing silane compound asa raw material and an amorphous silicon nitride powder beforecalcinating for crystallization in the imide thermal decompositionmethod. A nitrogen-containing silane compound and amorphous siliconnitride powder have the specific surface area value of from 300 to 850m²/g and contain a higher percentage of aggregated particles, in whichcoarse aggregated particles not less than 50 μm are contained in a rangeof not less than 10%. The presence of coarse aggregated particles are acause of simultaneously forming coarse particles by the abnormal graingrowth, needle-like crystal particles and fine aggregated particles,thereby forming particles uneven ingrain size and grain shape andprecluding preparation of a silicon nitride powder with the uniformgrain size. A silicon nitride powder for a mold release material with ahigher percentage of monodispersed primary particles of granularcrystals according to the present invention can be obtained by improvingthe grinding status of the coarse aggregated particles followed bycalcination. A method of reducing coarse aggregated particles includes amethod of grinding the aggregated particles in a state of an amorphoussilicon nitride powder and a method of changing the condition in athermal decomposition step of a nitrogen-containing silane compound. Asilicon nitride powder after crystallization obtained by going through astep for reducing the coarse aggregated particles before calcination forcrystallization in this manner contains a less percentage of coarseparticles, needle-like crystalline particles, and fine aggregatedparticles, thereby allowing for increasing the percentage ofmonodispersed primary particles of granular crystals without goingthrough a grinding step of aggregated particles.

When aggregated particles are ground in a state of an amorphous siliconnitride powder, the aggregated particles have to be ground until thereis no aggregated particle not less than 50 μm. As the grinding method apreferred method is a method in which the number of unused balls forgrinding in a vibrating ball mill is reduced to increase thevolume-based ball filling ratio of the mill to from 80 to 90%, at whicha ratio of the planetary movement of rotation and revolution isincreased. Grinding is generally performed by filling a continuous-typevibrating ball mill with resin-coated iron balls at the volume-basedball filling ratio of the mill from 60 to 70%, but this ordinary methodleaves a larger empty space in the vibrating mill making more balls movefreely, resulting in a longer time for balls for grinding to collideeach other leaving the aggregated particles of amorphous silicon nitridepowders with the particle diameter up to 300 μm in a particle sizedistribution. To the contrary, a method of reducing the number of unusedballs for grinding in a vibrating ball mill to increase the volume-basedball filling ratio of the mill to from 80 to 90%, at which a ratio ofthe planetary movement of rotation and revolution is increased canachieve a higher grinding efficiency to reduce the particle diameter ofamorphous silicon nitride powders below 50 μm in a particle sizedistribution. Exceeding the volume-based ball filling ratio of the millabove 90% is not preferred, since balls for grinding become harder tomove reducing the grinding efficiency. The number of needle-likecrystals and aggregated particles of crystals after crystallization canbe reduced by grinding the aggregated particles in amorphous siliconnitride powders with a higher grinding efficiency as compared to aconventional method, yielding silicon nitride powder with a higherdensity of a molded compact in which the percentage of monodispersedprimary particles of granular crystals is increased.

A silicon nitride powder prepared by the imide thermal decompositionmethod is fine particles, and while there is a mild grinding step toreduce aggregation, a material of resin-coated metal balls and a siliconnitride sintered compact are used as the grinding media to minimizecontamination with metal impurities to a very low level of not exceedingseveral ppm and to yield the powder suitable as a raw material for amold release material of a mold for casting a polycrystalline siliconnitride ingot. As described in JP 2007-261832 A, the power generatingefficiency of a solar cell is decreased by metal impurities contained ina silicon ingot, particularly it is said that its decrease of theefficiency due to contamination with multivalent metals is significantand the effects of the purity of a raw material powder itself as a moldrelease material on the power generating efficiency is also a commonknowledge of one skilled in the art so that it is said that lowercontamination with metal impurities is preferred as a raw materialpowder for a mold release material.

The percentage of monodispersed primary particles of granular crystalsof the silicon nitride powder obtained as described above can be readilydetermined by observing an SEM image of the powder at increase ofmagnification. Specifically, a sample powder is added to an acetonesolvent and dispersed by sonication bath to form a dilute acetonesolution of the sample, which drips on an SEM glass slide to dry,followed by vapor deposition of gold to obtain an SEM image, in whichdispersed particles can be observed. As similar to a method ofcalculating the percentage of a silicon nitride powder occupied in themold release layer described below, this SEM image can be used to tracea contour of a silicon nitride particle to calculate the area ratiousing an ImageJ software for image analysis.

A slurry containing a silicon nitride powder for a mold release layeraccording to the present invention is a slurry in which the siliconnitride powder for a mold release material is dispersed in water andapplied to the inner surface of a mold for casting a polycrystallinesilicon ingot to dry forming a mold release layer. A slurry containing asilicon nitride powder for a mold release layer according to the presentinvention can be obtained by adding the silicon nitride powder for amold release material to distilled water in a vessel, which is filledwith balls made of silicon nitride to mix and grind for a predeterminedtime using a grinding and mixing machine such as a vibrating mill, aball mill, and a paint shaker, or using an agitator with blades such aspaddle blades and a high-speed rotation and revolution type mixer whenballs are not used. A slurry containing a silicon nitride powder for amold release layer according to the present invention is applied to theinner surface of a quartz crucible with the porosity of from 16 to 26%or a quartz crucible accommodated into a graphite crucible as a moldusing a spray, a brush or a spatula and dried at the temperature of from30 to 120° C., followed by heat treatment under atmosphere at atemperature of from 800 to 1200° C. for a predetermined time to fix amold release layer to a mold.

A mold release material for casting a silicon ingot according to thepresent invention is a mold release material constituting the moldrelease layer and characterized by containing a silicon nitride powderfor a mold release material related to the present invention.

A mold for casting a silicon ingot according to the present invention isa mold in which the mold release layer is formed on the inner surfacethereof, and characterized in that the percentage of a silicon nitridepowder occupied in a mold release layer is high, since a silicon nitridepowder with a higher percentage of primary particles of granularcrystals monodispersed in the mold release layer is used. When siliconnitride particles with the area ratio of monodispersed primary particlesof granular crystals to be not less than 95% is used, the area ratio ofa silicon nitride powder occupied in the mold release layer is from 45to 60%. When the area ratio of monodispersed primary particles ofgranular crystals is no more than 95%, there are many needle-likecrystals and aggregated particles of fine particles resulting in thearea ratio occupied by the silicon nitride particles to be below 45%,reducing the bonding force among powders constituting the mold releaselayer, and lowering the adhesive strength of the mold release layer aswell as reducing adhesion to a crucible making the mold release layereasily peeled, brittle and easily exfoliated and broken, making asilicon melt penetrating into the mold release layer and adhering to theinner wall of a casting crucible, and generating fragments and breakagewhen releasing a solidified silicon ingot, thereby reducing a yield.When the area ratio of monodispersed primary particles of granularcrystals is not less than 95%, there are less fine particles with asmaller diameter which can penetrate into a space between particles sothat the area ratio occupied by silicon nitride particles does notexceed 60%.

EXAMPLES

In the following, specific examples will be illustrated to describe thepresent invention in more detail.

In examples, the content of metal impurities in an amorphous siliconnitride powder and a silicon nitride powder was quantitatively analyzedby the inductively coupled plasma atomic emission spectroscopy (ICP-AES)after high pressure decomposition with hydrofluoric acid. 0.2 g of asample was weighed and placed in a Teflon (Registered Trademark) bottletogether with nitric acid and hydrofluoric acid, which was sealed with acap to digest with the acid under high pressure, followed by addingsulfuric acid and heating to concentrate until fumes were generated, towhich pure water and hydrochloric acid were added and heated to dissolvesoluble salts, followed by quantitative analysis of metals by ICP-AES.

The percentage of a silicon nitride powder occupied in a mold releaselayer was determined by embedding with an epoxy resin a crucible, towhich a mold release material was applied, dried and calcined underatmosphere, taking an image of a cross-section of the mold release layerat 2000 magnification using a field emission scanning electronmicroscope (FE-SEM) as illustrated in FIG. 2( a), zooming the SEM imageto 400%, tracing a contour of silicon nitride particles within a 20 μmsquare as illustrated in FIG. 2( b), and calculating the area ratiousing an ImageJ software for image analysis. In the percentage ofprimary particles of granular crystals monodispersed in a mold releaselayer, in the same manner, a contour of monodispersed primary particlesin an SEM image was traced to calculate the area ratio using an ImageJsoftware for image analysis.

The percentage of monodispersed primary particles of granular crystalsof a raw material powder could be determined by adding the sample powderto an acetone solvent, which was dispersed by sonication bath to form adilute acetone solution of the sample, which was dripped on an SEM glassslide and dried, followed by vapor deposition of gold to take an SEMimage, in which dispersed particles could be observed. As similar to amethod of calculating the percentage of a silicon nitride powderoccupied in the mold release layer described above, this SEM image wasused to trace a contour of silicon nitride particles to calculate thearea ratio using an ImageJ software for image analysis.

When the area ratio was confirmed by this method, it was found that thepercentage of primary particles of granular crystals monodispersed in amold release layer is almost the same as the percentage of primaryparticles of granular crystals monodispersed in raw material powders.

Further, the adhesive strength between a mold release layer and a moldwas also evaluated as the peel strength in a simple peeling test using atape illustrated in FIG. 3. Specifically, a transparent resin adhesivetape 18 mm wide was adhered to the surface of a mold release layer toslowly peel and determine the area of peeled, which was compared as therelative ratio to the contact area for adhesion to evaluate the peelstrength. The area of peeled was determined by taking a picture of thetape after peeled, followed by using an ImageJ software for imageanalysis described above. In this case, the larger a percentage of thearea of peeled, the peel strength is lower. The peel strength of a moldrelease layer of which a silicon nitride powder for a mold releasematerial with the area ratio of monodispersed primary particles ofgranular crystals to be not less than 95% was used had the area ofpeeled to be no more than 40% in the adhesion peel test with a tape,indicating good adhesive strength was obtained. When the area of peeledexceeded 40%, the mold release layer was brittle and easily peeled orbroken, and a silicon melt could penetrate into a mold release layer toadhere to the inner wall of a casting crucible and generate fragmentswhen releasing a solidified silicon ingot, thereby reducing a yield.

The status of a silicon melt penetrating into a mold was evaluated, asillustrated in FIGS. 4 and 5, by observing an FE-SEM image of across-section of the crucible bottom after a silicon melt test to assessthe status of a silicon melt penetrating into a crucible. FIG. 4 is anSEM image of a cross-section of a mold release layer near the cruciblebottom at 100 magnification using an FE-SEM, wherein the FE-SEM specimenwas prepared by applying, as a mold release material, a silicon nitridepowder with the specific surface area of 11.8 m²/g and the density of agreen compact pressed at uniaxial pressure of 2 tons/cm² to be 51.8%according to Example 3 to a quartz crucible with a dimension of a 5 cmsquare and a depth of 4 cm and dried, followed by heat treatment underatmosphere at 1,100° C. for 4 hours and addition of 75 g of siliconmetal granules to the crucible, which was kept under an Ar gasatmosphere at 1,450° C. for 1 hour, and then cooled to pull out, andreleasing a silicon ingot from a crucible, which was embedded with anepoxy resin similarly to FIG. 2( a), whereas FIG. 5 is an FE-SEM imageof a cross-section of a mold release layer around the same cruciblebottom, which was prepared by using an SN-E10 silicon nitride powderwith the specific surface area of 11.0 m²/g and the density of a greencompact pressed at uniaxial pressure of 2 tons/cm² to be 49.5% relatedto Comparative Example 4.

Example 1

At first, a silicon nitride powder for a mold release material relatedto Example 1 was prepared as follows. A toluene solution of a silicontetrachloride at concentration of 30% by volume was reacted with liquidammonia to synthesize silicon diimide, which was washed with liquidammonia and dried yielding silicon diimide. An amorphous silicon nitridepowder was obtained by thermally decomposing the silicon diimide at 900°C. under a stream of a mixed gas of air and nitrogen (oxygenconcentration in a mixed gas was 2% by volume) at a flow rate of 70L/hour per 1 kg of the powder.

The amorphous silicon nitride powder thus obtained by thermaldecomposition had a large specific surface area of from 300 to 850 m²/gand was obtained in a form of a powder, in which the content of coarseaggregated particles not less than 50 μm in a particle size distributionwas about 10%. Aggregated particles of the amorphous silicon nitridepowder obtained were ground in a high-efficiency continuous vibratingmill for grinding, to which metal balls for grinding coated with a resinwere added to adjust the volume-based filling ratio of the mill to from80 to 90%, yielding an amorphous silicon nitride powder with thediameter of aggregated particles to be below 50 μm in measurement of aparticle size distribution and with metal impurities contaminated to bebelow 5 ppm by reducing friction between a powder and a metal in areactor material and a device for handling the powder.

The amorphous silicon nitride powder was placed in a carbon crucible andheated to elevate the temperature from ambient temperature to 1,100° C.in 3 hours, from 1,100 to 1,400° C. at 50° C./hr, and from 1,400 to1,550° C. in 2 hours and kept at 1,550° C. for 1 hour, and then cooledto pull out. The amorphous silicon nitride powder obtained was subjectedto a mild grinding process of aggregated particles in a continuousvibrating mill, to which metal balls for grinding coated with a resinwere added to adjust the volume-based filling ratio of the mill to from70 to 80% yielding a silicon nitride powder for a mold release materialaccording to Example 1.

The specific surface area of the crystalline silicon nitride powderobtained was measured by a surface analyzer of FlowSorb 2300 made ofShimazu Corporation to give 5.5 m²/g. The density of a green compactpressed at uniaxial pressure of 2 tons/cm² was 53.5%, whereas the arearatio of monodispersed primary particles of granular crystals was 98%.

Tests of evaluating a mold release layer including measurement of thepeel strength of a mold release layer, measurement of the area ratio ofmonodispersed silicon nitride particles in a cross-section of a moldrelease layer, assessment of the status of a silicon melt penetratinginto a crucible, and assessment of the status of a silicon ingotreleased from a mold were performed using the silicon nitride powder fora mold release material.

At first, 10 g of the silicon nitride powder for a mold release materialobtained, 40 g of distilled water, and 100 g of silicon nitride ballswith the diameter of 10 φmm were placed in a 100 cc polyethylene bottleto seal with a cap, which was fixed to a vibrating mill to vibrate atthe amplitude of 5 mm and the frequency of 1,780 spm for 5 minutes formixing to prepare a 20% by weight aqueous slurry.

The 20% by volume aqueous slurry described above was sprayed to coat aquartz crucible with the porosity of 16% and the dimension of 5 cmsquare and the depth of 4 cm preheated at 40° C. and a quartz plate madeof the same material, and a cycle of drying at 40° C. and coating wasrepeated 4 times. The quartz crucible and the quartz plate after coatingwere dried at 40° C. overnight. After drying, a mold release layer wasglazed to the quartz crucible using a box-type electric furnace byelevating the temperature to 1,100° C. in 4 hours under atmosphere andmaintaining the temperature at 1,100° C. for 4 hours, followed bylowering the temperature. Thickness of each mold release layer in thecrucible and the plate was 150 μm in an average of five measuringpoints.

The quartz plate to which the mold release material was glazed was usedto determine the peel strength, the density of silicon nitride for themold release layer, and the percentage of monodispersed primaryparticles of granular crystals. As illustrated in FIG. 3, a transparentresin adhesive tape 18 mm wide was adhered to parts of a mold releaselayer of the quartz plate glazed with a mold release material, which wasslowly peeled to determine the percentage of the area of peeled relativeto the contact area for adhesion. Measurement of the area of peeled wasperformed by taking a picture of the tape after peeled, followed by useof an ImageJ software for image analysis described above. The percentageof the area of peeled was 20%.

The density of silicon nitride for a mold release layer and thepercentage of monodispersed primary particles of granular crystals weredetermined by embedding parts of the glazed quartz plate with an epoxyresin, of which a cross-section was cut to take an SEM image of across-section of the mold release layer at 2000 magnification using anFE-SEM, zooming the SEM image to 400% to independently trace a contourof whole silicon nitride particles within a 20 μm square and a contourof only monodispersed primary particles of granular crystals, followedby calculating the area ratio using an ImageJ software for imageanalysis. The area ratio of whole silicon nitride particles was 50%,whereas the percentage of monodispersed primary particles of granularcrystals was 98%.

A quartz crucible glazed with a mold release layer was filled with 75 gof 99.999% pure silicon granules with a size of from 2 to 5 mm, whichwas heated, using a box-type electric furnace, under a stream of an Argas and an atmospheric pressure to elevate the temperature to 1,000° C.in 3 hours and from 1,000 to 1,450° C. in 3 hours and maintained at1,450° C. for 4 hours, followed by lowering the temperature. Afterlowering the temperature, the crucible was pulled out from the furnaceto observe the releasing status of a mold and visually observe thestatus of a silicon melt penetrating into the center of the cruciblebottom, further followed by embedding the center of the crucible bottomwith an epoxy resin, which was cross-sectionally cut to take an SEMimage of cross-section of a mold release layer at 100 magnificationusing an FE-SEM for observing the penetrating status of a silicon melt.The evaluation results are shown in Table 1. Symbols for the status of asilicon melt penetrating into a crucible in Table 1 mean that a doublecircle symbol indicates the silicon melt stopped its penetration at thesurface of the mold release layer, a circle symbol indicates the siliconmelt stopped its penetration within the mold release layer, a trianglesymbol indicates a small amount of a silicon melt was penetrated intoparts of the quartz crucible, and a cross symbol indicates a siliconmelt was penetrated through the quartz crucible to leak to the oppositesurface of crucible. Symbols for the status of a silicon ingot releasedfrom a mold mean that a double circle symbol indicates the silicon ingotwas completely released from a mold without adhesion to any parts of thequartz crucible, a circle symbol indicates the silicon ingot wasslightly adhered to the quartz crucible but released from a mold, atriangle symbol indicates the silicon ingot was released from the sideof the quartz crucible but adhered to its bottom, and a cross symbolindicates the silicon ingot was adhered to a whole quartz crucible andnot released from a mold.

TABLE 1 Evaluation of mold release layer Area of monodispersed Status ofparticles of molten silicon nitride liquid of in a cross- silicon Areaof section of penetrating Status of silicon peeled mold release into aingot released Class (%) layer (%) crucible from a mold Example-1 20 98◯ ◯ Example-2 15 96 ◯ ⊚ Example-3 10 95 ◯ ⊚ Example-4 5 95 ◯ ⊚ Example-525 98 ◯ ◯ Example-6 30 99 ◯ ◯ Example-7 40 97 ◯ ◯ Comparative 30 80 X ΔExample-1 Comparative 25 70 Δ Δ Example-2 Comparative 40 50 X XExample-3 Comparative 25 55 Δ Δ Example-4 Comparative 85 95 X XExample-5

Example 2

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide similarly prepared to Example 1 at 900° C.under a stream of a mixed gas of air and nitrogen (oxygen concentrationin a mixed gas was 2.5% by volume) at a flow rate of 70 L/hour per 1 kgof the powder. The amorphous silicon nitride powder obtained was groundunder similar condition to Example 1 in a high-efficiency continuousvibrating mill for grinding, to which metal balls grinding coated with aresin were added to adjust for the volume-based filling ratio of themill to from 80 to 90%. The amorphous silicon nitride powder aftergrinding was placed in a carbon crucible and heated to elevate thetemperature from ambient temperature to 1,100° C. in 3 hours, from 1,100to 1,400° C. at 50° C./hr, and from 1,400 to 1,550° C. in 2 hours andkept at 1550° C. for 1 hour, followed by cooling to yield a crystallizedsilicon nitride powder. The crystallized silicon nitride powder obtainedwas subjected under similar condition to Example 1 to a mild grindingprocess of aggregated particles in a continuous vibrating mill, to whichmetal balls for grinding coated with a resin were added at thevolume-based filling ratio of the mill from 70 to 80%, yielding amonodispersed silicon nitride powder for a mold release materialaccording to Example 2. The specific surface area of the silicon nitridepowder for a mold release material obtained was 8.5 m²/g, the density ofa green compact pressed at uniaxial pressure of 2 tons/cm² was 52.4%,and the area ratio of monodispersed primary particles of granularcrystals was 96%. A method similar to Example 1 was used to evaluate themold release layer in which the crystalline silicon nitride powder wasused. The evaluation results are shown in Table 1.

Example 3

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide similarly prepared to Example 1 at 1,050° C.under a stream of a mixed gas of air and nitrogen (oxygen concentrationin a mixed gas was 3% by volume) at a flow rate of 70 L/hour per 1 kg ofthe powder. The amorphous silicon nitride powder obtained was groundunder similar condition to Example 1 in a high-efficiency continuousvibrating mill for grinding, to which metal balls for grinding coatedwith a resin were added to adjust the volume-based filling ratio of themill to from 80 to 90%. The amorphous silicon nitride powder aftergrinding was placed in a carbon crucible and heated to elevate thetemperature from ambient temperature to 1,100° C. in 3 hours, from 1,100to 1,400° C. at 60° C./hr, and from 1,400 to 1,550° C. in 2 hour andkept at 1,550° C. for 1 hour, followed by cooling to yield acrystallized silicon nitride powder. The crystallized silicon nitridepowder obtained was subjected to a mild grinding process under similarcondition to Example 1 to yield a monodispersed silicon nitride powderfor a mold release material according to Example 3. The specific surfacearea of the silicon nitride powder for a mold release material obtainedwas 11.8 m²/g, the density of a green compact pressed at uniaxialpressure of 2 tons/cm² was 51.8%, and the area ratio of monodispersedprimary particles of granular crystals was 95%. A method similar toExample 1 was used to evaluate the mold release layer in which thecrystalline silicon nitride powder was used. The evaluation results areshown in Table 1.

FIG. 4 also illustrates the status of a silicon melt penetrating into amold in which the silicon nitride powder for a mold release materialaccording to Example 3 was used. It was found from FIG. 4 that a whitesilicon melt stops its penetration at the mold release layer.

FIG. 6 further illustrates the status of a solidified silicon ingotreleased from a mold after the silicon melting test in Example 3. It wasfound from FIG. 6 that a silicon melt did not penetrate into or adhereto a mold, indicating good releasing properties of the silicon ingot.

Example 4

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide powder similarly prepared to Example 1 at1,050° C. under a stream of a mixed gas of air and nitrogen (oxygenconcentration in a mixed gas was 3.5% by volume) at a flow rate of 70L/hour per 1 kg of the powder. The amorphous silicon nitride powderobtained was ground under similar condition to Example 1 in ahigh-efficiency continuous vibrating mill for grinding, to which metalballs for grinding coated with a resin were added to adjust thevolume-based filling ratio of the mill to from 80 to 90%. The amorphoussilicon nitride powder after grinding was placed in a carbon crucibleand heated to elevate the temperature from ambient temperature to 1,100°C. in 3 hours, from 1,100 to 1,400° C. at 80° C./hr, and from 1,400 to1,550° C. in 2 hours and kept at 1550° C. for 1 hour, followed bycooling to yield a crystallized silicon nitride powder. The crystallizedsilicon nitride powder obtained was subjected to a mild grinding processunder similar condition to Example 1 to yield a monodispersed siliconnitride powder for a mold release material related to Example 4. Thespecific surface area of the silicon nitride powder for a mold releasematerial obtained was 13.0 m²/g, the density of a green compact pressedat uniaxial pressure of 2 tons/cm² was 51.3%, and the area ratio ofmonodispersed primary particles of granular crystals was 95%. A methodsimilar to Example 1 was used to evaluate the mold release layer inwhich the crystalline silicon nitride powder was used. The evaluationresults are shown in Table 1.

Example 5

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide similarly prepared to Example 1 at 800° C.under a stream of a mixed gas of air and nitrogen (oxygen concentrationin a mixed gas was 1% by volume) at a flow rate of 70 L/hour per 1 kg ofthe powder. The amorphous silicon nitride powder obtained was groundunder similar condition to Example 1 in a high-efficiency continuousvibrating mill for grinding, to which metal balls for grinding coatedwith a resin were added to adjust the volume-based filling ratio of themill to from 80 to 90%. The amorphous silicon nitride powder aftergrinding was placed in a carbon crucible and heated to elevate thetemperature from ambient temperature to 1,100° C. in 3 hours, from 1,100to 1,400° C. at 30° C./hr, and from 1,400 to 1,550° C. in 2 hours andkept at 1,550° C. for 1 hour, followed by cooling to yield acrystallized silicon nitride powder. The crystallized silicon nitridepowder obtained was subjected to a mild grinding process under similarcondition to Example 1 to yield a monodispersed silicon nitride powderfor a mold release material according to Example 5. The specific surfacearea of the silicon nitride powder for a mold release material obtainedwas 3.0 m²/g, the density of a green compact pressed at uniaxialpressure of 2 tons/cm² was 55.1%, and the area ratio of monodispersedprimary particles of granular crystals was 98%. A method similar toExample 1 was used to evaluate the mold release layer in which thecrystalline silicon nitride powder was used. The evaluation results areshown in Table 1.

Example 6

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide similarly prepared to Example 1 at 700° C.under a stream of a mixed gas of air and nitrogen (oxygen concentrationin a mixed gas was 0.5% by volume) at a flow rate of 70 L/hour per 1 kgof the powder. The amorphous silicon nitride powder obtained was groundunder similar condition to Example 1 in a high-efficiency continuousvibrating mill for grinding, to which metal balls for grinding coatedwith a resin were added to adjust the volume-based filling ratio of themill to from 80 to 90%. The amorphous silicon nitride powder aftergrinding was placed in a carbon crucible and heated to elevate thetemperature from ambient temperature to 1,100° C. in 3 hours, from 1,100to 1,400° C. at 30° C./hr, and from 1,400 to 1,550° C. in 2 hours andkept at 1,550° C. for 1 hour, followed by cooling to yield acrystallized silicon nitride powder. The crystallized silicon nitridepowder obtained was subjected to a mild grinding process under similarcondition to Example 1 to yield a monodispersed silicon nitride powderfor a mold release material according to Example 6. The specific surfacearea of the silicon nitride powder for a mold release material obtainedwas 1.1 m²/g, the density of a green compact pressed at uniaxialpressure of 2 tons/cm² was 55.9%, and the area ratio of monodispersedprimary particles of granular crystals was 99%. A method similar toExample 1 was used to evaluate the mold release layer in which thecrystalline silicon nitride powder was used. The evaluation results areshown in Table 1.

Example 7

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide similarly prepared to Example 1 at 500° C.under a stream of a mixed gas of air and nitrogen (oxygen concentrationin a mixed gas was 0.5% by volume) at a flow rate of 70 L/hour per 1 kgof the powder. The amorphous silicon nitride powder obtained was groundunder similar condition to Example 1 in a high-efficiency continuousvibrating mill for grinding, to which metal balls for grinding coatedwith a resin were added to adjust the volume-based filling ratio of themill to from 80 to 90%. The amorphous silicon nitride powder aftergrinding was placed in a carbon crucible and heated to elevate thetemperature from ambient temperature to 800° C. in 3 hours, from 1,100to 1,400° C. at 15° C./hr, and from 1,400 to 1,550° C. in 2 hours andkept at 1,550° C. for 1 hour, followed by cooling to yield acrystallized silicon nitride powder. The crystallized silicon nitridepowder obtained was subjected to a mild grinding process under similarcondition to Example 1 to yield a monodispersed silicon nitride powderfor a mold release material according to Example 7: The specific surfacearea of the silicon nitride powder for a mold release material obtainedwas 0.5 m²/g, the density of a green compact pressed at uniaxialpressure of 2 tons/cm² was 56.3%, and the area ratio of monodispersedprimary particles of granular crystals was 97%. A method similar toExample 1 was used to evaluate the mold release layer in which thecrystalline silicon nitride powder was used. The evaluation results areshown in Table 1.

Comparative Example 1

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide powder prepared similarly to Example 5 at800° C. under a stream of a mixed gas of air and nitrogen (oxygenconcentration in a mixed gas was 1% by volume) at a flow rate of 70L/hour per 1 kg of the powder. The amorphous silicon nitride powderobtained was ground in an ordinary-efficiency continuous vibrating millfor grinding, to which metal balls for grinding coated with a resin wereadded to adjust the volume-based filling ratio of the mill from 60 to70%. Since the amorphous silicon nitride powder after grinding remainedat a low level of grinding as compared to Example 1, the percentage ofaggregated particles was high in a range of not less than 10% for coarseaggregated particles with a size of not less than 50 μm in measurementof a particle size distribution, and aggregated particles of 200 μm atmaximum were contained. The amorphous silicon nitride powder aftergrinding was placed in a carbon crucible and heated to elevate thetemperature from ambient temperature to 1,100° C. in 3 hours, from 1,100to 1,400° C. at 30° C./hr, and from 1,400 to 1,550° C. in 2 hours andkept at 1,550° C. for 1 hour, followed by cooling to yield acrystallized silicon nitride powder related according to ComparativeExample 1. The crystallized silicon nitride powder obtained wassubjected to a mild grinding process under the similar condition toExample 1 to yield a silicon nitride powder. The specific surface areaof the silicon nitride powder obtained was 3.2 m²/g, the density of agreen compact pressed at uniaxial pressure of 2 tons/cm² was 53.0%, andthe area ratio of monodispersed primary particles of granular crystalswas 90%. A method similar to Example 1 was used to evaluate the moldrelease layer in which the crystalline silicon nitride powder was used.The evaluation results are shown in Table 1.

Comparative Example 2

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide prepared under the similar condition toExample 2 at 900° C. under a stream of a mixed gas of air and nitrogen(oxygen concentration in a mixed gas was 2.5% by volume) at a flow rateof 70 L/hour per 1 kg of the powder. The amorphous silicon nitridepowder obtained was ground in an ordinary-efficiency continuousvibrating mill for grinding, to which metal balls for grinding coatedwith a resin were added to adjust the volume-based filling ratio of themill to from 60 to 70%. The percentage of aggregated particles in theamorphous silicon nitride powder after grinding was high in a range ofnot less than 10% for aggregated particles with a size of not less than50 μm in measurement of a particle size distribution, and aggregatedparticles of 250 μm at maximum were contained. The amorphous siliconnitride powder after grinding was placed in a carbon crucible and heatedto elevate the temperature from ambient temperature to 1,100° C. in 3hours, from 1,100 to 1,400° C. at 50° C./hr, and from 1,400 to 1,550° C.in 2 hours and kept at 1,550° C. for 1 hour, followed by cooling toyield a crystallized silicon nitride powder according to ComparativeExample 2. The crystallized silicon nitride powder obtained wassubjected to a mild grinding process under similar condition to Example1 to yield a silicon nitride powder. The specific surface area of thesilicon nitride powder obtained was 8.1 m²/g, the density of a greencompact pressed at uniaxial pressure of 2 tons/cm² was 51.6%, and thearea ratio of monodispersed primary particles of granular crystals was85%. A method similar to Example 1 was used to evaluate the mold releaselayer in which the crystalline silicon nitride powder was used. Theevaluation results are shown in Table 1.

Comparative Example 3

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide powder prepared under the similar conditionto Example 1 at 1,050° C. under a stream of a mixed gas of air andnitrogen (oxygen concentration in a mixed gas was 5% by volume) at aflow rate of 70 L/hour per 1 kg of the powder. The amorphous siliconnitride powder obtained was ground in an ordinary-efficiency continuousvibrating mill for grinding, to which metal balls for grinding coatedwith a resin were added to adjust the volume-based filling ratio of themill to from 60 to 70%. The percentage of aggregated particles in theamorphous silicon nitride powder after grinding was high in a range ofnot less than 10% for aggregated particles with a size of not less than50 μm in measurement of a particle size distribution, and aggregatedparticles of 270 μm at maximum were contained. The amorphous siliconnitride powder after grinding was placed in a carbon crucible and heatedto elevate the temperature from ambient temperature to 1,150° C. in 3hours, from 1,100 to 1,400° C. at 80° C./hr, and from 1,400 to 1,550° C.in 2 hours and kept at 1,550° C. for 1 hour, followed by cooling toyield a crystallized silicon nitride powder according to ComparativeExample 3. The crystallized silicon nitride powder obtained wassubjected to a mild grinding process under similar condition to Example1 to yield a silicon nitride powder. The specific surface area of thesilicon nitride powder obtained was 14.0 m²/g, the density of a greencompact pressed at uniaxial pressure of 2 tons/cm² was 50.5%, and thearea ratio of monodispersed primary particles of granular crystals was50%. A method similar to Example 1 was used to evaluate the mold releaselayer in which the crystalline silicon nitride powder was used. Theevaluation results are shown in Table 1.

FIG. 7 also illustrates the status of the solidified silicon ingotreleased from a mold after a silicon melt test in Comparative Example 3.It was found from FIG. 7 that the silicon ingot was adhered to a castingcrucible and prevented from release, indicating poor mold releasingproperties.

Comparative Example 4

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide prepared under similar condition to Example3 at 1,050° C. under a stream of a mixed gas of air and nitrogen (oxygenconcentration in a mixed gas was 3% by volume) at a flow rate of 70L/hour per 1 kg of the powder. The amorphous silicon nitride powderobtained was ground in an ordinary-efficiency continuous vibrating millfor grinding, to which metal balls for grinding coated with a resin wereadded to adjust the volume-based filling ratio of the mill to from 50 to60%. The percentage of aggregated particles in the amorphous siliconnitride powder after grinding was high in a range of not less than 10%for the aggregated particles with a size of not less than 50 μm inmeasurement of a particle size distribution, and aggregated particles of300 μm at maximum were contained. The amorphous silicon nitride powderafter grinding was placed in a carbon crucible and heated to elevate thetemperature from ambient temperature to 1,100° C. in 3 hours, from 1,100to 1,400° C. at 60° C./hr, and from 1,400 to 1,550° C. in 2 hours andkept at 1,550° C. for 1 hour, followed by cooling to yield acrystallized silicon nitride powder according to Comparative Example 4.The crystallized silicon nitride powder obtained was subjected to a mildgrinding process under similar condition to Example 1 to yield a siliconnitride powder. The specific surface area of the silicon nitride powderobtained was 11.0 m²/g, the density of a green compact pressed atuniaxial pressure of 2 tons/cm² was 49.5%, and the area ratio ofmonodispersed primary particles of granular crystals was 55%. A methodsimilar to Example 1 was used to evaluate the mold release layer inwhich the crystalline silicon nitride powder was used. The evaluationresults are shown in Table 1.

FIG. 5 also illustrates the status of a silicon melt penetrating into amold using the silicon nitride powder according to Comparative Example4. It was found from FIG. 5 that a white silicon melt penetrated intothe inside of a crucible.

Comparative Example 5

Next, an amorphous silicon nitride powder was obtained by thermallydecomposing silicon diimide powder prepared under similar condition toExample 1 at 500° C. under a stream of a nitrogen gas at a flow rate of70 L/hour per 1 kg of the powder. The amorphous silicon nitride powderobtained was ground in a high-efficiency continuous vibrating mill forgrinding, to which metal balls for grinding coated with a resin wereadded to adjust the volume-based filling ratio of the mill to from 80 to90%. The amorphous silicon nitride powder after grinding was placed in acarbon crucible and heated to elevate the temperature from ambienttemperature to 1,100° C. in 3 hours, from 1,100 to 1,400° C. at 10°C./hr, and from 1,400 to 1,550° C. in 2 hours and kept at 1,550° C. for1 hour, followed by cooling to yield a crystallized silicon nitridepowder according to Comparative Example 5. The crystallized siliconnitride powder obtained was subjected to a mild grinding process undersimilar condition to Example 1 to yield a silicon nitride powder. Thespecific surface area of the silicon nitride powder obtained was 0.3m²/g, the density of a green compact pressed at uniaxial pressure of 2tons/cm² was 57.0%, and the area ratio of monodispersed primaryparticles of granular crystals was 95%. A method similar to Example 1was used to evaluate the mold release layer in which the crystallinesilicon nitride powder was used. The evaluation results are shown inTable 1.

A silicon nitride powder SN-E10 made of Ube Industries, Ltd. used inPatent Literature 1 is a silicon nitride powder produced by an imidethermal decomposition method, but the amorphous silicon nitride powderas the raw material remains at a very low level of grinding and theamorphous silicon nitride powder after grinding contains coarseaggregated particles with a size of not less than 50 μm in a range of10% as the amorphous silicon nitride powder after grinding inComparative Example 4, in which coarse aggregated particles inmeasurement of a particle size distribution as illustrated in FIG. 8.Therefore, a large amount of aggregated particles remains in the siliconnitride powder after crystallization, which contains a large amount ofparticles other than monodispersed primary particles of granularcrystals. In production of a silicon nitride powder for a mold releasematerial according to the present invention, it is possible to preventthe formation of coarse aggregated particles with a size of not lessthan 50 μm by improving the grinding process as the amorphous siliconnitride powder after grinding in Example 3, and after a calcinationprocess for crystallization, a silicon nitride powder can be preparedsuch that the area ratio of monodispersed primary particles of granularcrystals is not less than 95%, when an ImageJ software for imageanalysis is used to analyze an SEM image of the powder. This makesuniform the density and thickness of the mold release layer of thesilicon nitride powder formed in a mold, increases the adhesive strengthamong particles in the mold release layer and between the mold releaselayer and the mold, and prevents cracking and peeling of the moldrelease layer, which causes mold release failure.

REFERENCE SIGNS LIST

-   1 Sample-   2 Core-   3 Die-   4 Lower die punch-   5 Upper die punch

1. A silicon nitride powder for a mold release material for a mold ofcasting a polycrystalline silicon ingot, wherein a percentage of primaryparticles of granular crystals monodispersed in the silicon nitridepowder are not less than 95% in terms of an area ratio calculated fromanalysis of a scanning electron microscope (SEM) image of the powder. 2.The silicon nitride powder for a mold release material for a mold ofcasting a polycrystalline silicon ingot according to claim 1, wherein aspecific surface area value is between 0.5 and 13 m²/g.
 3. A slurrycontaining a silicon nitride powder for a mold release layer for a moldof casting a polycrystalline silicon ingot, wherein the silicon nitridepowder for a mold release material for a mold of casting apolycrystalline silicon ingot according to claim 1 is dispersed inwater.
 4. A mold release material for casting a polycrystalline siliconingot, which contains the silicon nitride powder for a mold releasematerial for a mold of casting a polycrystalline silicon ingot accordingto claim
 1. 5. A mold for casting a polycrystalline silicon ingot, whichcomprises a mold release layer formed of the mold release materialaccording to claim 4, formed in the inner surface of a mold.